You can delete any transition between states. You cannot delete a state containing files or even one that has had files in it. Deleting a state would affect the history of the files. If all of the files that have ever been in the state are deleted and destroyed, thus removing their history, a state can be deleted.

What I've done in the past is to remove any transitions to the unwanted state and rename it "Do not use".

You can't delete used states, but you can rename them. So sometimes instead of creating a brand new state, you can reuse the old one you mothballed. Only downside is History becomes weird for some files.

This is also an example of why it is better to start out simple, and add some states later. Good luck.